In most communication system, not at least wireless communication systems, it is of highest importance to provide a reliable protocol for delivering data units from one entity to at least another entity in the system, without loss of data and without duplication of data. Such reliable data delivery protocols typically relies one the principle that the receiver of the data responds to the sender of the data with acknowledgements upon reception of the data and/or negative acknowledgements if the data units were lost. The sender will subsequently to the acknowledgement send the next data unit, or if a negative acknowledgement, retransmit the lost data unit.
Automatic repeat request (ARQ) is one of the most common retransmission techniques in communication networks, and ensures reliable user data transfer and data sequence integrity. The data is, prior to the transmission, divided into smaller packets, protocol data units (PDU). A reliable transfer is enabled by encoding packets with an error detecting code, such that the receiver can detect erroneous or lost packets and thereby order retransmission. The data sequence integrity is normally accomplished by sequential numbering of packets and applying certain transmission rules.
In the most simple form of ARQ, commonly referred to as Stop-and-Wait ARQ, the sender of data stores each sent data packet and waits for an acknowledgement from the receiver of a correctly received data packet, by the way of a acknowledgement message (ACK). When the ACK is received, the sender discard the stored packet and sends the next packet. An example of a prior art Stop-and-Wait ARQ scheme is schematically depicted in the message sequence chart of FIG. 1a. The process is typically supplemented with timers and the use of negative acknowledgement messages (NACK), which is illustrated in FIG. 1b. The sending entity uses a timer, which is started on the transmission of a data packet, and if no ACK has been received before the timer expires the data packet is retransmitted. If the receiver detects errors in the packet it can send a NACK to sender. Upon receiving the NACK the sender retransmit the data packet without waiting for the timer to expire. If the ACK or NACK message is lost, the timer will eventually expire and the sender will retransmit the data packet. From the simple Stop-and-Wait, more elaborated schemes of the conventional ARQ has been developed, for example Go-Back-N and Selective Reject (or Selective Repeat), which provides a higher throughput. Taught in WO 02/09342 by Dahlman et al. is a ARQ scheme that adds flexibility to the traditional ARQ scheme by introducing ARQ parameters that are set and/or negotiated to give a desired trade-off as regard to communication resources.
In another line of development of the ARQ, the redundancy in the coding is exploited in various ways to enhance communication performance (generally measure as throughput). These schemes are referred to as Hybrid ARQ schemes. Due to the combination of coding and ARQ, the hybrid ARQ schemes can give a certain adaptation to changes in the radio environment, e.g. to fading. How to best combine ARQ and coding schemes to cope with fading channels is not trivial. Several approaches and schemes have been suggested and used.
In Hybrid 1 ARQ Forward Error Correction (FEC) is combined with ARQ. In Hybrid 2 ARQ a PDU is sent more or less uncoded, but accompanied with a Cyclic Redundancy Check (CRC) for checking presence of bit errors after decoding. If CRC fails, i.e. errors are detected, then the PDU is requested for retransmission, and a codeword which is generated based on the data transmitted with the first PDU is sent. The codeword may have such character that the original dataword can be determined solely by decoding the codeword or it may be combined with the previously received PDUs content, and thereby improving the chance of decoding the data word without errors. The coding may for instance employ so-called half rate invertable codes. A version of Hybrid 2 ARQ is used in UMTS. Another Hybrid ARQ method is to combine a PDU that is transmitted multiple times by maximum ratio combining (or similar such as interference Rejection Combining). Disclosed in U.S. Pat. No. 6,308,294 by Ghosh et al. is a method of combining so called turbo codes with Hybrid ARQ, which allows retransmission of different sizes, and increased adaptability to fading channels.
As exemplified above advantages have been made in providing ARQ schemes that increases throughput and/or offer flexibility with regards to channel quality. However, the prior art methods suffers from drawbacks, mainly:
Conventional non-Hybrid ARQ schemes, but to some extent also Hybrid ARQ schemes, are inefficient when the channel quality changes unpredictable. Such changes may be caused by that the radio channel fluctuates due to fading, or that interference fluctuates unpredictable due to fading or/and due to unpredictable traffic fluctuations.
Further may the channel fluctuation cause inaccuracies in channel measurements and/or that outdated channel measurements are used for link mode selection. This may cause packets to be sent with a rate that is not decodable if the interference and noise is greater than permitted for the selected rate. Alternatively, a margin may be introduced and a reduced rate used, but this is done with the “cost” of not efficiently use the channels that can bear a higher rate.
In addition, complex ARQ schemes, especially the more advanced Hybrid ARQ schemes, are complicated to implement and require a high degree of optimization to fully take advantage of the increased throughput that is theoretically possible. Often the lack of system optimization, which has become very complex, makes the wireless systems deliver less throughput than the Hybrid ARQ schemes are capable off.